Japanese Patent Laid-Open Publication No. 8-113137 discloses a brake booster apparatus for a vehicle that is provided with a control valve to control the pressure balance between a constant pressure chamber and a variable pressure chamber. This apparatus includes a diaphragm that divides the interior of a housing into a constant pressure chamber and a variable pressure chamber, power piston, a valve plunger and a control valve. The control valve is defined by a vacuum control valve and an air control valve arranged in the axial direction. These control valves include a vacuum control seal (first seat portion) and an air control seal (second seat portion) respectively. The air control seal is interposed between a valve body and a valve seat which is formed at the input side edge portion of the valve plunger. The air control seal also includes a ring shaped plate.
When the air control seal is assembled into the valve body, the air control seal changes shape when it is mounted at a predetermined position. However, in this assembly process, the air seal is deformed and so a precise valve seat arrangement cannot be established. As a result, a reliable and effective air-tight arrangement cannot be produced.